Even though the LHC has not discovered any particles beyond the ones of the Standard Model (SM) yet, in the last years intriguing hints for the violation of lepton flavour universality (LFU) have been accumulated (see e.g. Refs. [1]}, [2]} for recent reviews). Among them, the updated measurement of \(R_{K^+}={\cal B}(B^+\rightarrow K^+\mu ^+\mu ^-)/{\cal B}(B^+\rightarrow K^+ e^+e^-)\)  [3]} by LHCb [4]} is particularly prominent since it provides first evidence for LFU violation (LFUV) in a single \(b\rightarrow s \ell ^+\ell ^-\)  transition. Furthermore, when combining all tests of LFUV in \(b\rightarrow s \ell ^+\ell ^-\)  [5]}, [6]}, [7]}, [8]} with other \(b\rightarrow s\mu ^+\mu ^+\)  observables, such as \(P_5^\prime \)  [9]}, [10]} and \(B_s\rightarrow \phi \mu ^+\mu ^-\)  [11]}, [12]}, one finds a preference for certain new physics (NP) hypothesis of more than \(7\,\sigma \)  [13]}
compared to the SM hypothesis.Very close results and pulls were found in the analysis of Ref. [14]} using also a complete set of observables but a different treatment of hadronic uncertainties and form factors. See also results in Ref. [15]} for an analysis using a smaller subset of the available data as well as Ref. [16]}, [17]}, [18]}, [19]} (see Ref. [20]} for a detailed comparison).
